The necromantic planeswalker Liliana's origin story is intrinsically tied to a character who's never gotten his own Magic card: one of the most powerful Demons in the Multiverse, Kothophed, who offered to increase her powers in exchange for her soul. Liliana showed him up by simultaneously selling that soul to three other Demons, then getting out of his part of the contract by killing him. But Kothophed's vengeance weighed heavily on her, and now she's doomed unless she offs the other three - half down now that she's snuffed Griselbrand, but still a long way to go.
The dark machinations of the Kothophed plot, despite appearing outside of a block story, proved popular, and when hakuplaneswalker asked Mark Rosewater on his Tumblr blog, "Kothophed was one of the most interesting demons of mtg. Why we don't have a Kothophed card?" Rosewater made it an open question.
The consensus? Kothophed needs to be in an upcoming supplemental product, because plenty of players want to make evil bargains with their Commanders. User drkobolds said "He was made for story reasons and killed in story. Sounds like another one for the great list of things to do in supplementary products," while sphinxofumculthum wrote "If we’re returning dead characters for supplemental products, I would much rather have something like Urza or Ugin… But it’s a nice Vorthos touch."